The invention is in the semiconductor fabrication field.
An important performance and reliability issue in many types of semiconductor devices is the quality of contacts. The resistivity of the contacts directly impacts speed, power consumption, and the tendency of a device to heat during operation. Metals, such as copper, make good contacts but bond poorly to semiconductors. Contact separation obviously limits device reliability. Accordingly, contacts are typically formed by metallizations in the art. The search for high quality contacts is a continuing effort in the art.
P-type Group III nitride materials form the basis for many important semiconductor devices, including transistors, light emitting diodes and laser diodes. For these devices to be efficient, contacts to the devices must be ohmic and exhibit low contact resistances. Absent extremely low contact resistances, contact electrodes will overheat and impact device lifetime. Overheated p-GaN ohmic contacts due to poor conductivity have been identified as the life limiting factor in nitride laser diodes. See, Nakamura et al., Appl. Phys. Lett. 62, 2930 (1993). Poor p-ohmic contacts have also limited performance in AlGaN/GaN heterojunction bipolar transistors. See, McCarthy et al, IEEE Electron Device Lett. 20, 277 (1999).
These types of problems have limited the development of optoelectronic devices based upon the p-type Group III nitrides. The difficulty in forming contacts to the p-type Group III materials arises from their large bandgaps and low hole concentrations resulting from difficulties in achieving high doping and the large ionization energy required to activate the magnesium acceptors.
Known past efforts have resulted in three ohmic metallization processes on moderately doped p-GaN that have achieved specific contact resistances less than 10xe2x88x924 xcexa9-cm. One method uses Ta/Ti bilayer contacts and requires a post-deposition anneal of 20 minutes at 800xc2x0 C. to achieve RC=3xc3x9710xe2x88x925 xcexa9-cm2 on p-GaN doped at 7xc3x971017 cmxe2x88x923. The electrodes produced by the method are unstable in air. Another method which achieves contact resistances as low as 4xc3x9710xe2x88x926 xcexa9-cm2, requires deliberate oxidation of Ni/Au contacts at 500xc2x0 C.xcx9c600xc2x0 C. for ten minutes. The oxidation is difficult to control and can result in the oxidation of unintended device areas. Another method forms platinum contacts on carefully cleaned GaN. The cleaning is conducted to remove all oxygen from the GaN surface prior to platinum metallization. Boiling potassium or aqua regia (3:1 mixture of HCl:HNO3) are often used for the cleaning operation. Apart from the inherent volatility and danger of such a process, the base tends to attack photo resist, and acid attacks other metal layers on the device.
Thus, there is a need for an improved metallization method to form ohmic contacts on the p-type Group III nitride material systems. The present invention is directed to this need.
The metallization method of the invention uses an oxide-forming metal layer to improve adhesion and getter surface contamination or oxides. A high work function metal is then formed on the oxide-forming layer. An anneal is conducted to diffuse the high work function metal through the oxide-forming layer. One or more metal cap layers may top the high work function metal to protect the high work function metal.
In the metallization method for forming a contact electrode on p-type Group III nitride materials, the oxide forming metal layer is deposited on a surface of a p-type Group III layer by, for example, electron beam evaporation. The oxide forming metal layer may comprise titanium, chromium, or palladium. Subsequently, a high work function metal q"PHgr"m greater than 4.5 eV is deposited on the oxide forming layer. An anneal is conducted at a temperature sufficient to diffuse the high work function metal through the oxide layer. A nonreactive metal layer may be deposited on the high work function metal as a cap/overlay to protect the contact from oxidation.